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Date:   Wed, 20 Feb 2019 18:23:27 +0800
From:   Herbert Xu <herbert@gondor.apana.org.au>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Nikolay Aleksandrov <nikolay@cumulusnetworks.com>,
        Thomas Graf <tgraf@suug.ch>,
        syzbot <syzbot+bc5ab0af2dbf3b0ae897@syzkaller.appspotmail.com>,
        bridge@lists.linux-foundation.org,
        David Miller <davem@davemloft.net>,
        LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Roopa Prabhu <roopa@cumulusnetworks.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: KASAN: use-after-free Read in br_mdb_ip_get
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On Mon, Jan 28, 2019 at 09:28:36AM +0100, Dmitry Vyukov wrote:
>
> > Weird, this is the kfree() on the error path of br_multicast_new_group()
> > when rhashtable_lookup_insert_fast() fails, which means the entry should
> > not be linked in the rhashtable or the hlist.
> > All other frees are via kfree_rcu. I'll keep looking..
> 
> Humm.... +rhashtable.c maintianers
> 
> The code in br_multicast_new_group is effectively:
> 
> mp = kzalloc(sizeof(*mp), GFP_ATOMIC);
> err = rhashtable_lookup_insert_fast(&br->mdb_hash_tbl, &mp->rhnode,
> br_mdb_rht_params);
> if (err)
>     kfree(mp);
> 
> So it looks like rhashtable_lookup_insert_fast both returned an error
> and left the new object linked in the table. Since it happened only
> once, it may have something to do with concurrent resizing/shrinking.

I've looked through the rhashtable code in question and everything
looks OK.  So I suspect some earlier corruption has occured to cause
this anomalous result.  Is it possible to collect earlier alloc/free
stack traces on the object in question?

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt